ECO LO GIC A L HEALT H Photo credit: Michael Pereckas, via http://tinyurl.com/hkzp9n5 under CC BY 2.0 license
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Chloride
The river meets standards for chloride, but levels are increasing throughout the metro area. 1 teaspoon of salt is enough to permanently pollute 5 gallons of water.
Chloride comes primarily from road deicing salt and water softeners.
Description and impacts. Chloride is an element that naturally occurs at low levels in Minnesota lakes and streams. However, at high concentrations, it can be toxic to aquatic life.1 High chloride levels can also affect groundwater and drinking water supplies, vehicles, infrastructure, soil and plants, pets and wildlife.2 Chloride is a permanent pollutant; it does not degrade over time, so it cannot be feasibly removed from surface waters.3
Figure 1. Sources of chloride to metro area waters other, including residential 4%
other/natural background 2%
water softeners 23%
commercial 14%
cities 25%
D E I C I N G S A LT S MnDOT 17%
counties 15%
Source: Alexander, et. al.67
Source: Minnesota Pollution Control Agency
39 local water bodies are impaired by excess chloride, including several river tributaries.
Sources. Winter road deicers are the primary source of chloride to Minnesota’s waters; approximately 350,000 tons of deicing chemicals are applied in the metro area each year.4 Seventy-eight percent of chloride applied in the Twin Cities area is retained in local surface and groundwater.5 Water softeners, which discharge chloride to septic systems or wastewater treatment facilities (which do not remove chloride), are another important source. Other sources include fertilizers, dust suppressants, and landfill leachate (Figure 1). Status. The river is currently well below the state’s chloride standards to protect aquatic life. However, many local tributaries, lakes and wetlands fail to meet these standards (see map and Figure 2). Of tested metro area water bodies, 39 are impaired for excess chloride with another 38 close to exceeding state standards.6 Monitoring data also show high chloride concentrations in area groundwater. Twenty-seven percent of the metro area sand and gravel wells exceed federal guidelines, as compared to 1% of non-metro wells.7,8 History and trends. Chloride levels in the metro river increased by 81% from 1985 to 2014 (Figure 3). Chloride concentrations are also increasing in metro area surface waters and groundwater. Cyanide formation. Some deicing salts may contain iron-cyanide compounds to prevent caking when exposed to moisture. These compounds can break down when exposed to sunlight to form free cyanide, which can be toxic